Plant Life Floods Earth's Atmosphere

A new study estimates that 80 to 90 percent of the atmospheric water vapor originating from Earth's continents comes from plant transpiration rather than simple physical evaporation. This process uses up almost half of the solar energy absorbed by our landmasses and represents a major piece of our terrestrial climate system. There may be implications for our investigations of other worlds.

The recent discovery of two potentially 'habitable', nearly Earth-sized, planets in a five-planet system around the very distant star Kepler-62 reinforces the fact that astronomers are edging closer and closer to finding worlds that have a chance of resembling our home world in some way. But there are so many unknowns that it's tremendously difficult to state with any certainty what the surface environment might be on such planets, much less what the odds are for life and a functioning biosphere. Still, what these discoveries do provide us with is a set of new questions.

For example, if we ever manage to study a potentially terrestrial-equivalent planet with enough fidelity to measure its atmospheric properties (and there's good reason to think we'll do this within the next decade), we need to know what to look for. Chemical disequilibrium is one fingerprint of life on a planet, but so too is the way the planet responds to what are called 'external forcings'. That's stuff like seasons - the varying energy input as the planet orbits its star, and as it spins about its axis - slowly or quickly.

Which is one reason a newly published result by Jasechko et al. in Nature is particularly fascinating for astrobiologists (see also this). The study, titled 'Terrestrial water fluxes dominated by transpiration', uses a wealth of data on the isotopic fractionation of oxygen and hydrogen in water on Earth's continental masses, along with some careful mathematical modeling, to learn about the processes by which water goes from being a liquid to a vapor in our atmosphere.

Electron microscope image of a stomata on the underside of a plant leave - a 'mouth' that allows water vapor to escape to the air.

In a nutshell, when water evaporates physically - like in a drying puddle - the molecules that turn to vapor consist preferentially of lighter isotopes (atomic nuclei with lower neutron counts), leaving behind the heavier stuff for a while. But when a plant takes up water, carries it through its structures, and breathes it out as vapor through its stomata - in other words transpires water - it doesn't seem to care whether the isotopes are light or heavy, and an equal mix goes into the atmosphere as is left behind as liquid.

So, in principle, this offers a way to keep track of how much water is put into our atmosphere by plant transpiration as opposed to simple evaporation. It's far from simple though, because water of course rains back onto the surface. But land-locked lakes offer a way to sample how the isotopic flavors all end up mixing together - because we have some hope of modeling how they get supplied and where the water goes.

The upshot is that it seems likely that 80 to 90 percent of the water that undergoes 'evapotranspiration' (the combination of simple evaporation and plant transpiration) is carried by transpiration alone. That's five to ten times more than via direct evaporation, and as much as four times more than previously thought. To put this in further perspective; it means that every year 62,000 cubic kilometers of water is moved by plant life from the continental surfaces of the planet into the atmosphere. That's a lotta water, the same as a giant droplet some 50 kilometers in diameter.

But the most interesting thing from the point of view of observing how a planet functions, is the energy involved. This study estimates that roughly half of the solar power absorbed by the Earth's continental surface goes into driving plant transpiration. That's half of 70 Watts per square meter on the planet's landmasses, which add up to about 5,000 Terawatts of power (5x10^15 Watts, annually averaged).

Thus, not only is a huge piece of the Earth's hydrological machinery driven by life, it's quite the power hog. Us filthy humans use energy at a rate of about 15 Terawatts, which is barely 0.016% of the average 89 Petawatts of solar power absorbed by all land and oceans. Transpiration could account for a much more substantial 5.6%.

If correct, this has some profound implications for the way in which climate models handle biologically driven water transport. It may also have something to tell us about the response of a life-rich world to varying stellar input, and what we might expect to detect in its atmosphere. Since transpiration is affected by a host of physical characteristics of both the plants themselves and the environment - from the number of leaves, to local temperatures and wind conditions - it's tempting to imagine that one day we might be able to deduce some of these details on an exoplanet by simply monitoring the water vapor in its atmosphere.

Of course that assumes many things about the commonality of vascular plant life across the cosmos, which is a wee bit of a stretch since such forms didn't arise on Earth until about 400 million years ago, but we can still speculate that mechanisms like transpiration could evolve elsewhere.

Caleb Scharf is the director of Columbia University's multidisciplinaryAstrobiology Center. He has worked in the fields of observationalcosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.)
Follow on Twitter @caleb_scharf

The views expressed are those of the author and are not necessarily those of Scientific American.

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